Current emission control regulations necessitate the use of catalysts in the exhaust systems of automotive vehicles in order to convert carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NOx) produced during engine operation into unregulated exhaust gasses. Vehicles equipped with diesel or another lean burn engine offer the benefit of increased fuel economy, however, control of NOx emissions in such systems is complicated due to the high content of oxygen in the exhaust gas. In this regard, Selective Catalytic Reduction (SCR) catalysts, in which NOx is continuously removed through active injection of a reductant, such as urea, into the exhaust gas mixture entering the catalyst, are know to achieve high NOx conversion efficiency. A typical lean exhaust gas aftertreatment system may also include an oxidation catalyst coupled upstream of the SCR catalyst. The oxidation catalyst converts hydrocarbons (HC), carbon monoxide (CO) and nitrous oxide (NO) in the engine exhaust gas. The oxidation catalyst is also used to supply heat for fast warm up of the SCR catalyst.
The inventors herein have recognized several disadvantages with such system configuration. Namely, because the oxidation catalyst is typically located under-body far downstream of the engine, it takes a significant time to reach light-off temperatures (e.g. 200 deg. C.). This results in delayed warm up for the SCR catalyst, and thus negatively affects emission control. Also, since the oxidation catalyst does not convert the incoming hydrocarbons until it reaches light-off, under some conditions, such as cold start, or extended periods of light load operation, hydrocarbons may slip from the oxidation catalyst and cause the SCR catalyst poisoning.